1. Field of the Invention
The present invention relates to a method for controlling an air/fuel ratio of an internal combustion engine, and more particularly to a method by which an air/fuel ratio of a mixture to be supplied to the engine is controlled using feedback towards a target value in response to an output signal level of an oxygen concentration sensor.
2. Description of Background Information
Conventional methods for controlling the air/fuel ratio by feedback operation utilize oxygen concentration in the exhaust gas of the engine which is detected by an oxygen concentration sensor and the air/fuel ratio of mixture being supplied to the engine to approach a target value in response to an output signal level from the O.sub.2 sensor for the purpose of the purification of the exhaust gas and improvements of the fuel economy.
On the other hand, in engines equipped with a three-way catalytic converter in the exhaust system, it is conventional to control the air/fuel ratio of the mixture being supplied to the engine to a stoichiometric value (14.7) since, as shown in FIG. 1, an optimum operation of the three-way catalytic converter is attained when the air/fuel ratio of the supplied mixture is at around the stoichiometric air/fuel ratio. (Japanese Patent Application Laid open No. 59-201,946 and Japanese Patent Application Laid Open No. 60-19931 are examples of such a technique).
In the conventional technique of the air/fuel ratio control, a type of oxygen concentration sensor whose output signal level is not proportional to the oxygen concentration in the exhaust gas has been utilized. As shown in FIG. 2, the output signal characteristic of this type of oxygen concentration sensor is such that different two stable output levels appear on both sides of the stoichiometric air/fuel ratio, i.e., in the rich region and the lean region.
In the air/fuel ration control methods using the oxygen concentration sensor whose output signal level is not proportional to the oxygen concentration, it is possible to detect from the output signal level of the oxygen concentration sensor, whether the air/fuel ratio of the supplied mixture is on the rich side or on the lean side with respect to the stoichiometric air/fuel ratio. However, it is difficult to determine, from the output signal level of the oxygen concentration sensor, that the center of the air/fuel ratio of the supplied mixture is at the stoichiometric air/fuel ratio. Therefore, it is difficult to ascertain if the three-way catalytic converter is operating effectively when trying to attain a good exhaust gas purification operation.
It is also known that the purification rate of the three-way catalytic converter can be raised by a perturbation operation by which the air/fuel ratio is varied around the stoichiometric air/fuel ratio as a central value. Further, as shown in FIG. 3, the degree of the improvement of the purification rate is proportional to the frequency of the perturbation operation. In conventional methods of air/fuel ratio control, a perturbation effect is generated through the process of the feedback control. Therefore, the frequency of perturbation is determined by such factors as the moving time of the mixture and the exhaust gas, the delay in the response of the oxygen concentration sensor, and the operational time lag of the control system. More specifically, the frequency of the perturbation is increased as the period of each cycle of the feedback control reduces. In this cycle the air/fuel ratio of the mixture being supplied to the engine is adjusted on the air intake side; the oxygen concentration in the exhaust gas is detected by the oxygen concentration sensor; and the air/fuel ratio of the mixture being supplied to the engine is again controlled on the basis of the result of the detection of the oxygen concentration.
However, as shown in FIG. 4, the period of the feedback control varies in inverse proportion to the rotational speed of the engine, and as shown in FIG. 5, increases as the position of the oxygen concentration sensor in the exhaust gas passage is shifted on the downstream side. Therefore, even if the air/fuel ratio of the mixture is detected to be at the stoichiometric value accurately by the oxygen concentration sensor, it does not mean that the efficiency of the purification of the exhaust gas by the three-way catalytic converter meets the minimal requirements, since the frequency of the perturbation can not be raised to a high enough level.